1. Field of the Invention
This invention relates to a liquid crystal display (LCD) device, wherein a light shielding layer is formed on thin film transistors.
2. Background Art
A light shielding layer known as a black matrix has been provided in LCD devices. The pattern of the light shielding layer is designed so as to expose only the display area of display electrodes to improve contrast of displayed images.
The LCD device has first and second transparent insulating substrates, such as glass, spaced from each other, and liquid crystal material contained in the space. In a typical thin film transistor (TFT) type LCD device, the light shielding layer having a pattern for exposing only the display areas is formed on the first substrate, then a common electrode of Indium Tin Oxide (ITO) is formed, and an orientating layer, such as polyimide, is formed. A thin metal layer, such as Cr, can be used as the light shielding layer since the entire common electrode is maintained at a fixed reference potential.
On the second substrate, horizontal metal gate lines and vertical metal data lines are formed. At each crosspoint of the gate and data lines, a display cell is formed which includes a TFT and the display electrode of ITO. In addition, a passivation layer and an orientating layer are formed on the structure. Polarizers are arranged outside the first and second substrates, and a light source is provided to project the light to the LCD device. A data line driver and a gate line driver connected to the data and gate lines, respectively are selectively activated to apply a voltage to the liquid crystal between selected display electrodes and the common electrode in order to display an image. Since the light shielding layer is formed on the first substrate, known as the common electrode substrate, and the display electrodes are formed on the second substrate, known as the TFT substrate, both substrates must be carefully assembled to align apertures of the light shielding layer with the display electrodes.
To solve this alignment problem, it has been proposed that the light shielding layer be formed on the TFT substrate, but, this would cause another problem. Specifically, the metal light shielding layer cannot be used on the TFT substrate because the presence of the metal light shielding layer causes an undesirable capacitance effect across the insulating layer (such as the passivation layer), and the conductive elements (such as the data and gate lines).
To solve the problem, the inventors of the present invention have tried to use a photopolymer (i.e. photoresist material) as the light shielding layer on the TFT substrate. To realize an optical density which is required to sufficiently shield or block the light, a thick photoresist layer is required. FIG. 1 shows a plan view of the cell and FIG. 2 shows the cross-sectional structure of the second glass substrate GS viewed along line 2--2 of FIG. 1. The first glass substrate is not shown in FIG. 2. The gate line or gate electrode GE is formed on the second glass substrate. A gate insulating layer GI is formed, an amorphous Si layer AS which operates as a channel region of the TFT is formed, and a display electrode DE of ITO is formed. An N+ amorphous Si layer SL and an aluminum layer AL are formed on the amorphous Si layer to form a drain electrode at the right side of the channel of the TFT and a source electrode at the left side. The drain region extends from the data line, and the source electrode is connected to the display electrode DE. A passivation layer PL is formed to cover the structure. Next, a thick black photoresist layer or light sheilding layer LS is formed to expose the display electrode. Orientating layer OL is formed on the entire structure. The orienting layer is rubbed in one direction, as shown in FIG. 1, to align liquid crystal molecules in one direction when no voltage is applied, as is well known in the art. It is required that the liquid crystal molecules tilt in the same direction, as shown by the liquid crystal molecules B. However, the liquid crystal molecules A near the edge of the aperture of the light shielding layer located up stream with respect to the rubbing direction tend to tilt in a reverse direction, as shown in FIG. 2 . This is called reverse tilt. The reverse tilt occurs due to the large step H, which is caused by the thick photoresist layer.
FIG. 3 shows a simplified structure of the aperture of FIG. 2, without the passivation layer and the orientating layer. The reverse tilt occurs in an area defined by L located up stream with respect to the rubbing direction. A boundary between the normal tilt region and the reverse tilt region forms a kind of disclination line, and is called a reverse tilt disclination line. The light from the light source passes through the liquid crystal along the reverse tilt disclination line. A line L shown in FIG. 1 shows the continuous reverse tilt disclination line appearing along the upper side edge and the left side edge of the aperture (located at the up stream position with respect to the rubbing direction when no voltage is applied across the liquid crystal).
The reverse tilt disclination line raises a problem when the LDC device is operated in a normally white mode. In the normally white mode, the polarizers are crossed and the liquid crystal molecules are twisted. When no voltage is applied across the liquid crystal, the light from the light source passes through the LCD device, whereby the white color is displayed. When a voltage is applied, the light is blocked by the polarizers, and the black color is displayed. However, if reverse tilt occurs when the voltage is applied, the light from the light source passes through the reverse tilt disclination line, so that a white line appears in the black image, whereby it degrades contrast and increases an afterimage or residual image effect. In this manner, the display quality is remarkably degraded by the reverse tilt.
Japanese patent application 63-162521 (Published Unexamined Patent Application 02-13927) solves the problem of the reverse tilt occurring at corner portions of a rectangular display electrode of the LCD device by enlarging the corner portions of the display electrode, whereby the reverse tilt occurs in the enlarged areas of the display electrode. Thus, patent application 63-162521 shows an approach which is very different from the present invention for solving the reverse tilt problem.